In some chemical and manufacturing plants, there is a simultaneous need for both oxygen and ozone for the designated equipment or processes. For example, the manufacture of precious metals, like gold, silver, or copper, often requires oxygen for production when using refractory ores. High pressure oxygen is used during the pressure oxidation process to oxidize sulfide minerals to more soluble forms of metal species that free the precious metal for downstream processing.
The application of high pressure oxygen for the production of precious metals is well known (See FLEMING, C. A., 2010, Basic iron sulfate—a potential killer in the processing of refractory gold concentrates by pressure oxidation. Minerals & Metallurgical Processing 27, 2, 81-880). The high pressure oxidation is used to extract precious metals from refractory ores.
In addition, in some cases it may be useful to employ ozone in this process to oxidize, precipitate, and recover base metals from the ore during the processing. For example, it has been traditionally shown that ozone is a very effective oxidizing agent to convert metal ions into less soluble forms which can be separated by precipitation. The utility of ozone to oxidize, for example, Fe(II) to Fe(III) has been shown in 1965 (Conocchioli et. al., Journal of the American Chemical Society, 1965, 87 (4) pp 926-927). U.S. Pat. No. 7,152,741 discloses the use of ozone for metals separation during flotation. U.S. Pat. No. 7,789,332 also discloses the use of ozone to facilitate oxidation, precipitation, and separation of desired metal species. PCT Application No. AU2012/000058 discloses use of ozone to facilitate separation of cobalt and manganese from the nickel containing ore.
Finally, nitrogen can be used as a gas to affect flotation separation of the ground minerals. For example, U.S. Pat. No. 6,044,978 discloses the utility of nitrogen as the gas employed in flotation processes.
Thus, in certain metals production processes the availability of oxygen, ozone and perhaps even nitrogen is desirable. However, the prior art is silent on the most efficient manner to produce oxygen, ozone, and nitrogen at a given chemical or manufacturing plant where all of these gases are desired.
Ozone (O3) is a triatomic molecule consisting of three oxygen atoms that is most often produced from oxygen or air in an ozone generator, for example, that typically operates at low pressure (1-3 bara/100-300 kPa). Ozone is a strong oxidizing agent that finds applications in the disinfection of wastewater, removal of odor from drinking water, oxidation of metal species in aqueous solutions for selective precipitation of undesired metals, air purification, and also has beneficial use in the pulp and paper industry.
Generation of ozone is typically accomplished in an ozone generator in which an oxygen containing gas is passed through two electrodes separated by a dielectric and a discharge gap. When voltage is applied to the electrodes, electrons travel across the discharge gap. These electrons dissociate oxygen leading to the formation of ozone.
The feed to the ozone generator can be dry air, but the use of pure oxygen results in lower energy consumption and a higher ozone concentration exiting the generator. The ozone concentration of the ozone generator may reach 10 wt % (the remainder being oxygen) with oxygen as feed to the generator. Thus, in industrial applications it is typically preferred to feed the ozone generator with a feed stream high in oxygen content.
In many applications (e.g. waste water treatment), the ozone is sparged or bubbled into the waste water liquid stream. After the ozone has been decomposed through oxidation reactions, the gas exiting the liquid is relatively pure oxygen (90-98 wt %). The re-use of that oxygen increases the overall efficiency of the process.
A cryogenic or non-cryogenic oxygen generator may be used to supply the ozone generator with oxygen. Since the ozone generator only converts about 10% of the oxygen to ozone, to improve the overall efficiency of the process, the oxygen-rich waste stream after ozone utilization can be recycled back to the ozone generator to be reused. This recycling can reduce the size of the oxygen generator required for supply to the ozone generator. The recycling of waste oxygen to the ozone generator was disclosed in U.S. Pat. No. 3,856,671.
If high-pressure oxygen is required at the manufacturing plant (i.e., 5 to 40 bara/500 to 4,000 kPa) this is traditionally achieved by cryogenic distillation of high-pressure air. Alternatively, a non-cryogenic process can be employed (e.g., pressure swing adsorption or PSA) which produces oxygen at low pressure and that low-pressure oxygen must be then compressed to yield the high-pressure oxygen.
Therefore, in circumstances where there is both the need for high-pressure oxygen and low-pressure ozone, traditional methods would require two air separation units. Air separation units are expensive and costly to run and maintain. Thus, there is a need in the art for an improved equipment design and process that would require only one air separation device for the production of both low-pressure ozone and high-pressure oxygen.